Secondary batteries refer to batteries capable of repeated charging and discharging. Main components of the secondary batteries include a cathode, an anode and electrolytic solution. The secondary batteries can be roughly classified according to the electrolytic solution. Among the secondary batteries, a secondary battery using an organic solvent for the electrolytic solution is referred to as a non-aqueous electrolyte secondary battery. A typical non-aqueous electrolyte secondary battery is a lithium-ion secondary battery.
The cathode of the non-aqueous electrolyte secondary battery usually contains an active material, an electrical conductive material, a binder, a current collector and the like.
The active material refers to an electrochemically active substance. Examples of the cathode active material of the non-aqueous electrolyte secondary battery include LiCoO2, LiNiO2, and LiMn2O4.
The cathode active material of the non-aqueous electrolyte secondary battery is categorized according to the structure of the active material. For instance, LiCoO2, LiNiO2 and the like are categorized into an active material of a layered structure. LiMn2O4 is categorized into an active material of a spinel structure.
LiMn2O4, which has the above spinel structure, is less expensive and safer than LiCoO2, LiNiO2 and the like having the layered structure. However, the energy density per a unit mass of the LiMn2O4 having the spinel structure is smaller than that of LiCoO2, LiNiO2 and the like having the layered structure. In order to increase the energy density per a unit mass, it is necessary to raise an operating voltage of a battery or to increase a discharge capacity.
In Patent Literature 1, a part of Mn in LiMn2O4 is substituted by Ni to raise the operating voltage of a battery. In one of the Examples, it is disclosed that an operating potential of a battery can be raised to 4.5 V or more by substituting a part of Mn in LiMn2O4 by Ni to produce LiMn1.5Ni0.5O4.
In Patent Literature 2, the discharge capacity of a battery is increased by providing an excessive Li in LiMn1.5Ni0.5O4. In one of Examples, it is disclosed that LiMn1.5Ni0.5O4 and LiI are reacted at 80 degrees C. for 13 hours to synthesize Li2Mn1.5Ni0.5O4, which shows a discharge capacity in a range of 160 to 180 mAh/g.
In Patent Literature 3, the discharge capacity of a battery is increased by reacting LiMn1.5Ni0.5O4 with LiI and subsequently annealing the reactant in a nitrogen atmosphere. In one of Examples, it is disclosed that Li2Mn1.5Ni0.5O4, which is synthesized by reacting LiMn1.5Ni0.5O4 and LiI and subsequently is annealed at 300 degrees C. for five hours in a nitrogen atmosphere, shows a discharge capacity of approximately 240 mAh/g.
These related arts are, though showing high energy density, insufficient in terms of deterioration of the discharge capacity after repeated charging and discharging (cycle characteristics).
Patent Literature 4 discloses that the cycle characteristics are improved by electrochemically inserting Li into LiMn1.5Ni0.5O4 to provide an excessive Li. However, though the method disclosed in Patent Literature 4 improves the cycle characteristics, sufficient discharge capacity cannot be obtained.